Former Russian spy Sergei Skripal and his daughter Yulia were poisoned with up to 100 milligrams of the nerve agent novichok, a chemical weapons watchdog has said.

Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons (OPCW), said the amount of novichok used suggests it was created for use as a weapon rather than for research purposes.

The poisoning left Mr Skripal, 66, and his 33-year-old daughter Yulia fighting for their lives after being found slumped on a bench at The Maltings shopping centre in Salisbury on 4 March.

Some 250 detectives have been working on the investigation into the attack in the Wiltshire city and have gone through more than 5,000 hours of CCTV and interviewed more than 500 witnesses.

Mr Uzumcu told the New York Times the novichok could have been applied as a liquid or aerosol.

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Video:Yulia Skripal thanks the people of Salisbury

He said: "For research activities or protection you would need, for instance, five to 10 milligrams or so, but even in Salisbury it looks like they may have used more than that, without knowing the exact quantity, I am told it may be 50, 100 milligrams or so, which goes beyond research activities for protection.

"It's not affected by weather conditions. That explains, actually, that they were able to identify it after a considerable time lapse."

He said the samples collected suggested the nerve agent was of "high purity".

Russia has vehemently denied that it was behind the attack and relations with the West have become seriously strained.

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Video:VR reconstruction of Russia spy's movements

Russian ambassador to the UK Alexander Yakovenko has suggested the Skripals could have been injected by British authorities with nerve agent produced at nearby Porton Down.

British authorities say only Russia had the means and motive to poison the former spy.

The UK's representative to the United Nations, Karen Pierce, told a meeting in April there was "no plausible alternative explanation than Russian State responsibility for what happened in Salisbury".

She said: "Russia has a proven record of conducting state-sponsored assassinations including on the territory of the United Kingdom.

"The independent inquiry into the death of Alexander Litvinenko concluded in January 2016 that he was deliberately poisoned with polonium; that the FSB had directed the operation; and that President Putin probably approved it."

She added: "No terrorist group or non-state actor would be able to produce this agent in the purity described by the OPCW testing and this is something Russia has acknowledged.

"The Russian State has previously produced novichoks and would still be capable of doing so today."